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Throughflow Data
Bonell (2005) refined the classical notions of temperate runoff production based on work to date in the tropics worldwide resulting in the following five categories: predominantly vertical pathways, lateral pathways - Hortonian / infiltration-excess runoff, lateral pathways - saturation-excess runoff, lateral pathways - subsurface stormflow within the soil, and lateral pathways - subsurface stormflow at the soil-bedrock interface. Based on the profile of hydraulic conductivity as a function of depth one should be able to predict which of the above processes is the likely dominant process. We are currently in the process of doing a preliminary soil survey, of course one must indeed verify the dominant process based on data. Besides soil moisture information the two methods that we can use to quantify sub-surface flux of water during a rainfall event are groundwater wells and throughflow troughs.
Throughflow Trough
We recently installed our first throughflow trench denoted by "tr" on the Gamboa Site Map. Sampling throughflow can be accomplished in a number of ways, we decided to dig a trench 3.0 m wide and about 1.2 m deep (weathered bedrock reached). PVC pipe cut with a slot is placed in the the trench and inclinated in such a way that all water drains toward a centrally located tipping bucket as shown by Fig 1. Remember that only saturated flow can be measured in such a manner.
Fig 1 View both inside and outside of the throughflow collection trough, the plastic sheet seen on the left photo was utilized to gain better contact between the PVC and the face of the trench.
Groundwater Wells
Shallow groundwater wells were hand augured to a depth of up to 1.5 m at five seperate locations on the Cerro Pelado hillside as indicated by "g1-g5" on the Gamboa Site Map. These groundwater wells were not meant as an attempt to measure any deep groundwater circulations as that would likely mean greater depths and drilling through bedrock. Rather they are meant to measure the evolution of any shallow or perched water tables that would contribute to event subsurface flow. We use Mini-trolls which are manufactured by In-Situ Inc to directly measure the pressure (and subsequently depth) of water in each monitoring well.
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